II. Field of Invention
The present invention relates to an anti-fouling paint generally used to top-coat exposed to a marine environment. These anti-fouling paints prevent the settlement and growth of marine organisms such as barnacles, algae and other organisms.
II. Description of the Prior Art
An anti-fouling coating is applied to control or prevent the attachment and growth of fouling organisms on surfaces immersed in sea-water. The growth of these organisms on submerged surfaces can lead to a dramatic rise in fuel costs. The build-up of fouling increases the frictional resistance of a ship's hull and drag effects resulting in considerably greater fuel consumption due to the increased weight and reduction in speed of the ship.
One of the successful anti-fouling paints developed in recent years is an anti-fouling paint using self-polishing copolymer. U.S. Pat. No. 3,167,473 and British Patent No. 1,457,590 describe self-polishing copolymers formed from triorganotin salt of an olefinically unsaturated carboxylic acid and at least one olefinically unsaturated co-monomer, into which other biocides/pigments are incorporated. The copolymer provides some of the biocidal action of the paint and hydrolyzes in sea water at a constant leaching rate.
However, ecological concerns surrounding organotin compounds released from tin-containing self-polishing copolymers makes the tin-based anti-fouling paints undesirable. Organotin has been implicated in failures of certain shellfish crops. Specifically, spawning failures and strange shell deformities have been noticed in adult oysters. It may be concluded that organotins are not only extremely toxic, but may further causes harmful side effects to a wide variety of non-target species, including both ecologically and commercially important organisms.
Therefore, recent advances in tin-free anti-foulant protection merit further investigation. Japanese Patent Publication No. 78/21885 describes a partially gelled binder made by mixing an acrylate copolymer containing acid and a metal ester such as zinc acetate and the like. However, such has inferior film-forming properties and storage stability, and therefore, exhibits problems such as easy cracking and peeling of the formed paint film due to crosslinking between resin binders caused by the metal ester. Moreover, application of such resin is rendered difficult because of its high viscosity.
Japanese Patent Application No. 81/165922 and Japanese Patent Application No. 83/196900 describe a class of polyester resins having a number of metal-ester bonds in their polyester backbone chains. Such resins are easily dissolved in an alkaline solution such as sea water at the metal-ester bonds of the polyester backbone and decomposed into small molecules. These decomposed small molecules cause the resin to have inferior film-forming qualities which makes the coating when it forms subject to easy cracking, peeling and flaking.
European Patent Application No. 204456 describes a hydrolysable resin which is used to improve the film-forming property of polyester resin having metal-ester bonds. The hydrolysable resin consists of a resin having at least one side chain bearing at least one terminal group of the formula:
wherein X represents ##STR1##
M is a metal selected from zinc, copper and tellurium; x is an integer of 1 or 2; R represents an organic acid residue selected from: ##STR2##
R1 is a monovalent organic residue.
Such resin may be prepared by a method wherein a polymerizable unsaturated monomer having the desired organic acid metal ester bond at an end portion is first prepared and copolymerized with other polymerizable unsaturated monomer(s).
However, this resin has some disadvantages: First of all, the viscosity of the resin binder containing a metal character is too high; such resin is also unstable during storage. In particular, it causes the increase of Tg and lacks durability because of its brittleness and tendency to crack easily in the form of paint film. The brittleness and cracking tendency result because the metal, M, in the hydrolysable resin has an extra d-orbital which forms a coordination complex with polar groups like the carbonyl group in the resin. The second disadvantage of the resin is that its application on the pre-coated tin-based anti-fouling paint is restricted by its poor recoatability, that is, poor adhesion with the tin-based anti-fouling paint. Its poor recoatability is caused by the hydrophobic property of an organic acid having a long-chain alkyl group. The third disadvantage is that the above mentioned resin erodes at too fast a rate during the initial period of erosion tests. This is because the resin has the ionic bonding character of metal which typically erode at a high rate in sea water compared to a tin-based resin which has a covalent bonding character. Finally, paint films containing this kind of resin often exhibit a flaking phenomenon due to the high erosion rate occurring during the initial period. Furthermore, in European Patent No. 204456 the acid value of the acid-polymer is limited up to 40 KOH mg/g. This value is inadequately low for obtaining long-term anti-fouling properties. The low acid value of a binder means that hydrolyzable portion of binder, after releasing the metal ester, is insufficient. Consequently, the paint surface is blocked by fouling organisms. As a result, the erosion rate after the initial period is dramatically decreased and then the paint loses its anti-fouling property.
WO 91/15546 and U.S. Pat. No. 5,199,977 describe metal-containing binders which comprise a substantially non-volatile amine, alcohol, urea, nitrile, or phenol as a ligand for inducing metal stability and low viscosity of the binder. The amine used in the coating composition preferably includes 12 to 20 carbon atoms such as a rosin amine. The use of amine as a ligand can control the erosion rate at the initial period and resolve problems of metal stability and high viscosity of the binder by forming a coordination complex with the metal. However, the use of the amine also results in low erosion due to its low solubility in water compared to that of a hydrolysable binder. In particular, using the amine as a ligand simply results in a dramatic increases of the erosion rate at the high pH of sea water, that is, an unstable erosion phenomenon wherein cracks appear in the paint film above the sea water line after long term exposure. The use of alcohol, urea, nitrile and phenol as a ligand results in poor water resistance which results in the flaking phenomenon characterized by a swelling of the paint film and color change thereof.